In general, three problems exist in connection with the deposition of thin films on surfaces. These problems are uniformity of film thickness, step coverage of the film over severe geometrical structures on the surfaces, and control of film morphology. When an adatom arrives at the surface of a growing film, it does not necessarily remain at its original point of impact but may migrate for a considerable distance depending upon the surface structure and the nature of the growing film, the temperature of the growing film, and the angle of incidence of the adatom.
Deposition from a spatially distributed deposition source gives a family of arrival angles at the surface to be coated which depends upon the geometry of the eroding target, the geometry of the features of the surface to be metallized, and the source to substrate orientation. In prior art apparatus, the eroding target has a predetermined shape. Typically, it is an inverse conic with a cone angle of the order of 30.degree.. In such a case, the spatial distribution of the deposited material is fixed throughout the deposition cycle.
As is well known in the prior art relating to planetary tooling, the representation of angled arrival by serial cyclic distribution of these angles with the same weighting factor is not the same as those angles represented in distribution all at the same time. This is due to the generation of self-shadowing structures on the growing film near topographically severe structures, such as near unity aspect ratio channels and contact windows. As a result of self-shadowing, the film is not properly deposited in such channels or windows.
To eliminate the self-shadowing or step coverage problem with thin films, heat and bias have been used in the past to increase the mobility of the adatoms. Since angles are represented all at the same time, the problem is severe and a large heating time is needed to gain acceptable step coverage. Unfortunately for this approach, the same mechanism which allows heat to solve the step coverage problem causes large grain sizes and considerable variation in grain size. These faults in film morphology are most troublesome regarding pattern delineation and electromigration. Similarly, RF bias is often required to provide adequate enhancement of step coverage and this presents severe problems, namely reduced deposition rate, grain size variation and contamination of the growing film by material deposited by the system side walls.
Because of the step coverage problems associated with conventional deposition techniques used with workpieces, such as semiconductor wafers, a need exists for improvements in the apparatus and the method of coating workpieces. The present invention provides such improvements.